In a supercritical heat pump cycle system using CO2 as refrigerant, for example, a gas cooler is used for heating a fluid, and an externally driven decompression device such as an electrical expansion valve is provided for controlling the operation state of the cycle system. However, in this case, a pressure sensor for detecting a refrigerant pressure and a control circuit for driving the electrical expansion valve are required, thereby increasing the cost.
When a mechanical expansion valve is used, a heat radiating amount of the gas cooler becomes larger when the outside air temperature is low. In this case, a refrigerant temperature at an outlet of the gas cooler decreases and a control pressure of the expansion valve decreases. Therefore, temperature of air to be blown from the gas cooler is greatly decreased.
Further, when a heating operation is performed in a supercritical heat pump cycle system using CO2 as refrigerant, even when a low-pressure refrigerant pressure decreases, the mechanical expansion valve is not opened until a high-pressure refrigerant pressure reaches a valve-open pressure, in order to control the high-press refrigerant pressure. Therefore, if the mechanical expansion valve is used as the decompression device of the supercritical heat pump cycle system, the low-pressure refrigerant pressure decreases at a time immediately after a refrigerant cycle start because the mechanical expansion valve is closed at the refrigerant cycle start.
FIG. 8A shows an example with a bad start condition and FIG. 8B shows an example with a good start condition, when a mechanical expansion valve of a comparison example is used in a super-critical heat pump cycle system. In this case of FIG. 8A, when the outside air temperature Tam becomes equal to or lower than −10° C. (e.g., −20° C. in FIG. 8A), the saturated refrigerant pressure becomes lower, and the pressure (i.e., the suction pressure of the compressor) of the low-pressure refrigerant at a refrigerant-cycle start time becomes lower. In this case, the high-pressure refrigerant pressure discharged from the compressor does not reach a valve-open pressure of the mechanical expansion valve. Accordingly, the flow amount of refrigerant flowing through the heat pump cycle system becomes almost zero, and heating capacity with the heat pump cycle system may be not obtained.